1. Field of the Invention
This invention relates to a microphone amplifier in a telephone station being furnished with current from the telephone network.
2. Description of the Prior Art
Microphone amplifiers are used in telephone stations for example in connection with electrodynamic microphones, since the level of the signals generated by the microphone is insufficient for transmission via the subscriber line to the exchange. Amplifiers of this kind are usually operated with the feeding current on the subscriber line furnished by the exchange.
When such an amplifier is provided with capacitive degenerative feedback a certain time has to pass before the operating point is reached after the amplifier is put into operation i.e. after the supply current is applied. An amplifier as described above is disclosed in the German Patent Application (Offenlegungsschrift) Ser. No. OS 28 12 735 which is similar to the amplifier circuit described in U.S. Pat. No. 3,899,742 dated Aug. 12, 1975.
During the charging period of the capacitor the amplifier acts like a resistor of high value, since the amplifier draws a very small current. This behavior causes a false operation during the dial process (pulse or multifrequency dialing), since after the transmission of a digit from the dial transmitter the subscriber line is switched to the speaking circuit (microphone) for taking over the line current immediately. As long as the amplifier has a high impedance the line current can only be led via the line balancing network. Because of the higher impedance of the latter the line current cannot reach its normal value. This may cause under certain operational circumstances a condition which may be recognized in the telephone exchange as open line.